A number of methods have heretofore been used to make steel parts. These methods often employ cold forming techniques, such as upsetting, heading and extrusion, which are well known in the art. In upsetting, the cross-sectional area of a portion or all of a blank of metal is increased. Heading is a particular form of upsetting where the blank is a wire, rod or bar stock. The heads of bolts are often made using heading techniques. In extrusion, the metal blank is forced through a die orifice of desired cross-sectional outline to produce a length of uniform cross section.
One such method for making high-strength steel parts which is well known begins by annealing or otherwise softening the steel blank. The annealed steel blank is then cold formed, in a process which includes one of the above types of forming techniques, into a desired geometric configuration. The now formed part is then heat treated, i.e., austenitized, hardened by quenching followed by tempering, to obtain the desired high-strength mechanical properties. The steel material of the resulting part has a tempered martensite microstructure. The mechanical properties produced from such heat treatments are often inconsistent and can vary widely from part to part. In addition, the annealing and heat treating steps significantly add to the cost of the overall process for making the high-strength steel parts, due in large part to the energy consumption associated with heating the part and the required labor and processing.
In another method for making such high-strength steel parts, the blank of steel is initially austenitized, hardened by quenching followed by tempering to the point where the mechanical properties of the post-heat treated blank are such that the blank can be subsequently cold formed, in a process which includes one of the above three forming techniques, into a desired geometric configuration. The steel material of the finished part from this method also has a tempered martensite microstructure. While this method apparently has advantages over the previously described method in that narrower strength tolerances from part to part have reportedly been obtained, this method still employs a costly heat treating process.
Cold forming blanks of high-strength material is known. In U.S. Pat. No. 3,904,445 issued to the present inventor, a method is disclosed for cold forming a length of high-strength steel bar stock into a U-bolt. The '445 patent discloses such a length of bar stock made of a steel material having a composition consisting essentially of, by weight percent: carbon between about 0.50-0.55%, manganese between about 1.20-1.65%, vanadium between about 0.03-0.05%, with the balance substantially all iron. However, cold forming a bend in a length of bar stock is less severe than other cold forming techniques, such as upsetting and extruding. Until this invention, it was thought that cold forming a blank of high-strength into a part by upsetting or extrusion type techniques would likely result in the formation of cracks or even fractures in the finished high-strength steel part or at the least would likely require the gradual formation of the part by a series of cold forming steps with an annealing or stress relieving step performed between successive cold forming operations. Such cracks or fractures would likely ruin the part. In addition, employing such cold forming and annealing steps would add to the time and cost of making such high strength steel parts.